Document EP-A-0 250 290 describes an example of such a hinge mechanism which gives complete satisfaction. In that hinge mechanism, there is an asymmetrical offset between two locking members relative to a third locking member, so that the second set of teeth on each of the two angularly offset locking members come to engage partially in the first set of teeth on the second cheek plate. That angular offset between two slugs makes its possible to take up operating clearance between the first cheek plate and the second cheek plate in order to improve the comfort of an occupant seated in a seat equipped with such a mechanism. However, that asymmetrical offset between the two locking members prevents the teeth on the two locking members from ever coming fully into engagement with the teeth in the set of teeth on the second cheek plate, which gives rise to loss of strength for the hinge mechanism as a whole.
In view of the tendency for safety standards to become increasingly strict, it is essential to increase further the strength of hinge mechanisms of the type mentioned above, in order to prevent, as far as possible, such mechanisms from breaking in the event of an accident, in particular when said hinge mechanisms are subjected to rotary torque that is particularly high.